Web of Dreams: Spiders Appear To Enter an REM-Like Sleep State

A new study that peered at the translucent bodies of juvenile spiders has found evidence to suggest they may undergo a form of rapid eye movement (REM) sleep, a state closely associated with dreaming in mammals.

The study’s authors, Paul Shamble and Daniela C. Rößler, who conducted the research while at Harvard University, got the idea for the study when they analyzed the sleep behavior of a species of jumping spider (Evarcha arcuata). The snoozing spiders suspended themselves on a silk thread through the night. Shamble and Rößler were curious as to what the arachnids were getting up to in this position. “We recorded the adults as they hung there. We saw these regularly occurring bursts of activity that just looked or reminded us a lot about when you watch cats and dogs sleep or dream where you just see these quite uncontrolled twitches,” Rößler told the Harvard Gazette.

See-through bodies reveal sleep signs

The team exploited the fact that jumping spiders’ babies have translucent bodies to scan for signs of REM sleep. While movable eyes have only evolved in a select number of lineages (insects, for example, do not have moving eyes), spiders possess retinal tubes that allow them to alter their gaze. When the spiders’ limbs twitched, so did their eyes. Given the fundamental differences in how spiders and mammals have evolved, Shamble tells the Gazette, this discovery raises as many questions as it answers. “In terms of significance behind that, that possibility makes you wonder what dreaming is for and what it’s doing,” he says.

A sleeping jumping spider. Credit: Harvard Gazette

Another characteristic feature of REM sleep is a relaxation of the muscles called atonia. This is, says Rößler, “basically motor function being greatly diminished so someone doesn’t run off while they dream.”

The sleeping spiders also appeared to show a kind of atonia, although their musculature is fundamentally different from our own. Spiders, rather than using muscles to stretch their limbs, instead use muscles in their heads to hydraulically pump fluid into their limbs, extending them. The sleeping spiders’ limbs were curled in, which the team believe is indicative of the spiders’ head muscles being shut off to avoid any night-time scuttling.

What does a spider dream of?

The researchers were, of course, unable to ask their arachnid participants whether they were actually dreaming while they twitched away, but the team write that they hope their findings can lead to answers about the function of both REM sleep and vision in the brain.

Asked by the Gazette what the spiders might be dreaming about, Shamble can only speculate: “It’s actually a question about the nature of dreams, which is sort of astonishing. You, sort of, just have to base it on your own experience. I assume that they’re dreaming about their own lives, like what happens to them during the day, the same way that we do — some strange visual version of their own experience. That’s pretty profound,” he concludes.

Reference:

Rößler DC, Kim K, De Agrò M, Jordan A, Galizia CG, Shamble PS. Regularly occurring bouts of retinal movements suggest an REM sleep–like state in jumping spiders. PNAS. 2022;119(33):e2204754119. doi:10.1073/pnas.2204754119

This article is a rework of a press release issued by Harvard University. Material has been edited for length and content.